Buffer zone for counterflow preheater

ABSTRACT

A steam generator having a U-shaped tube bundle, which extends upwardly from a tubesheet, and a wrapper encircling the tube bundle, a preheater disposed on the cool leg of the bundle and a distribution plate disposed between the tube sheet and the preheater to form a buffer zone, the buffer zone being effective to prevent cold influent feedwater entering the steam generator from channeling along the tubes and contacting the tubesheet.

United States Patent [191 Bennett 1 Nov. 4, 1975 BUFFER ZONE FOR COUNTERFLOW PREHEATER [75] Inventor: Robert R. Bennett, Tampa, Fla.

[73] Assignee: Westinghouse Electric Corporation,

Pittsburgh, Pa.

22 Filed: Jan. 15, 1974 21 Appl. No.: 433,615

52 us. C1 122/32; 165/112 [51] Int. Cl. F22b 1/06 [58] Field of Search 122/32, 33, 34; 165/112 [56] References Cited UNITED STATES PATENTS 3,483,848 l2/1969 Green 122/32 3,547,084 12/1970 Sprague 122/32 Romanos 122/32 Bennett 122/34 Primary Examiner-Kenneth W. Sprague Attorney, Agent, or Firm-F. J. Baehr 57 ABSTRACT A steam generator having a U-shaped tube bundle, which extends upwardly from a tubesheet, and a wrapper encircling the tube bundle, a preheater disposed on the cool leg of the bundle and a distribution plate disposed between the tube sheet and the preheater to form a buffer zone, the buffer zone being effective to prevent cold influent feedwater entering the steam generator from channeling along the tubes and contacting the tubesheet.

9 Claims, 6 Drawing Figures 13 --l5 17 12 ll US. Patent N0v.4, 1975 Sh eetlof2 3,916,843

US. Patent Nov. 4, 1975 Sheet 2 of2 3,916,843

I v u w u 6/ PM /////VK I k BUFFER ZONE FOR COUNTERFLOW PREHEATER BACKGROUND OFTHE iNvENTioN SUMMARY OF THE INVENTION In general, a vapor generator which utilizes a primary heated fluid to vaporize a secondary fluid, when make in accordance with this invention, comprises a vertical shell portion, a tubesheet, a plurality of tubes through which theprimary fluid flows, a wrapper encircling the tubes and forming an annular chamber between the wrapper and the shell, a secondary fluid inlet nozzle, a preheater having an enclosure disposed on the tubesheet end thereof, a flow distribution plate disposed between the preheater and the tubesheet. The flow distribution has a plurality of circulation openings disposed therein; The flow'distribution plate is cooperatively associated with the-closure plate and the wrapper to form a buffer zone between the preheater and the tubesheet to prevent influent secondary fluid that is introduced into the preheater from channelling along the tubes and contacting the tubesheet.

BRIEF DESCRIPTION OF THE DRAWINGS The objects and advantages of this invention will become more apparent from reading the following detailed description in connection with the accompanying drawings in whichcorresponding reference characters indicate corresponding portions throughout the draw ings and in which:

FIG. Us a partial sectional view of a steam generator made in accordance with this invention;

FIG. 2 is an enlarged partialsectional view showing a preheater and buffer zone in detail;

FIGS. 3, 4 and 5 are enlarged partial sectional views taken on lines III-III, lVIV and V V, respectively, in FIGS. 2 and 6;

FIG. 6 is a partial sectional view of an steam generator showing an alternate embodiment of this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawings in detail, FIG. 1 shows a steam or vapor generator 1, which utilizes a U-shaped bundle of tubes 3 to provide the heating surface required to-transfer heat from a primary fluid to vaporize or boil a secondary fluid. The steamgenerator 1 comprises a vessel 5 having a vertically oriented tubular shell portions 7 and an end closure or flanged and dished'head 9'enclosing one end of the shell,the upper end, and a spherical shaped channel head 11 enclosing the other end of the shell, the lower end. A tubesheet 13 is made integral with the channel head 11 and has a plurality of holes 14 disposed therein to receive the ends of the U-shaped tubes 3. A dividing plate 15 is centrally disposed in the channel head 11 to divide the channel head into two compartments 17 and 19, which serve as headers for the tubes 3. The compartment on the. left, as shown in FIG; 1, is the primary fluid inlet compartment 17 and has aprimary fluid inlet nozzle 21 in communication therewith. The compartment on the right, as shown in FIG. 1, is the primary fluid outlet compartments 19 and has a primary fluid outletnozzle (not shown) in communication therewith, thus causing the primary inlet fluid to flow through the tubes thereby creating a hot leg portion 23, the portion shown on the left in the drawings, and a cool leg portion 25, the portion shown on the right in the drawings. A secondary fluid or feedwater inlet nozzle 27 is disposed in the lower portion of the shell 7 adjacent the tube sheet 13 and has a thermal sleeve 29 disposed therein.

The tube bundle 3 is enclosed by a wrapper 31 which encircles the tube bundle and forms an annular chamber 33 adjacent the shell 7.

A preheater 35 is disposed within the shell 7 adjacent tubesheet 13. The preheater 35 is disposed to enclose at least a portion of the tube bundles and comprises a portion of the wrapper 31, a vertical closure plate 37, a top end closure plate 39, and a lower end closure plate 41, which cooperate to enclose a portion of the cool leg to form the preheater 35. Baffles 43 are disposed within the preheater 35 to cause the influent primary fluid to follow a sinuous path therethrough. A flow distribution plate 45 is disposed between the lower enclosure plate 41 and the tubesheet 13. The flow distribution plate 45 cooperates with the end closure 41 and the wrapper 31 to form a buffer zone 47 which is closed on all sides except one. j

A shown in FIG. 3 the flow distributor plate 45 has a plurality of large openings 49 surrounding each tube. The openings 49 are sufficiently large to provide open area to allow the secondary fluid to flow from adjacent the tube sheet 13 into the buffer zone 47.

As shown in FIG. 4 the end closure plate 41 has a plurality of holes 51 which tightly embrace the tubes 3. The holes 51 are sized to provide just enough open area to allow the tubes to slide freely therethrough.

As shown in FIG. 5, the top end closure plate 39 has a plurality of holes 53, which tightly embrace the tubes and a plurality of flow distribution holes 55 which allow the secondary fluid to flow from one side of the baffle to the other. e

The flow distribution plate 45 is. generally circular generally covering the whole cross-section of the wrapper 31. The flow distribution plate 45 has a semicircular portion through which the tubes forming the-hot leg 23 pass and the central portion of this semicircular section contains a large open area 57 which accommodates a plurality of the hot leg tubes 23 to prevent excessive boiling and low velocities adjacent the tubesheet 13 which causes chemical hideout and temperature excursionsthat have in the past produced premature tube failures. I

The steam generator 1 also has means for separating water or secondary fluidfrom the steam or vapor disposed in the upper portion of the shell which has an enlarged cross-section. Fora more complete description of the steam generator, reference may be made to an earlier application filed by the inventor, Feb. 9, 1972 and assigned Ser. No. 224,804, now U. S. Pat. No. 3,804,069. This application is assigned to the same assignee and is hereby incorporated by specification.

The annular space33 between the wrapper 31 and the shell 7 provides a recirculation path for the secondary fluid removed from'the vapor by the moisture separation means disposed at the top of the vessel. This recirculated secondary fluid flows downwardly to the area between the tubesheet and the flow distribution plate 45.'A portion of it flows upwardly through the opening 57 into the central portion of the hot leg sweeping the tubesheet and preventing boiling adjacent thereto; Another portion of this recirculated secondary fluid flows upwardly through the openings 49 into the buffer zone 47 because of the different size in the opening adjacent the tubesin the flow distribution plate 45 and the end closure plate 41. The recirculating secondary fluid sweeps upwardly along the tubes and into the buffer zone preventing influent secondary fluid, which is introduced into the preheater and flows through the holes 51, from channeling along the tubes and reaching the tubesheet. Thus, reducing thermal shock to the tubes adjacent the tubesheet and preventing premature tube and tubesheet failure. i The steam generator 1' shown in FIG. 6 incorporates the same U-shaped tube bundle 3, channel head 11 and integral tubesheet 13, however a second fluid outlet nozzle 59 is shown. The shell 7' is generally the same diameter throughout its length and a dividing plate 61 extendsupwardly from the tubesheet and cooperates with the wrapper 31 to form a steam or vapor end closure 63 forming a superheater portion around at least a portion of the hot leg portion 23 of the tube bundle 3. The wrapper 31 has a plurality of openings 65 disposed therein and a plurality of closure strips 63 which causejthe preheated secondary fluid to flow through a portion of the annular chamber 33 to the area between the'tube'sheet and the flow distribution plate 45, which in FIG. 6 is generally semicircular and extends between the wrapper and the dividingplate 61. As can be best seen in FIG. 3, the dividing plate 45 has a plurality of large openings 49surrounding the tubes '3. The openings are sufficiently large to allow area for the secondary fluid to flow upwardly therethrough into the buffer zone'47' while the lower closure plate 41 as shown in FIG. 4 has'a'plurality of holes which tightly embrace the tubes allowing only sufficient clearance for the plate to slide over the tubes. Thus, the preheated secondary fluid flows across the cool leg portion of the tubesheet and upwardly through. the openings 49 into the buffer zone sweeping the tubes and preventing influent feedwater or secondary fluid from channeling along the tubes and contacting the tubesheet. The preheated feedwater flows upwardly over the cool leg portion of the tube bundle 25, boils and becomes vapor or steam, it then follows a sinuous path over at least a portion of the hot leg portion of the tube bundle 23 to bereference in this come superheated vapor or steam and then'flow out- 4 water from contacting the tubesheet, excessive thermal stresses are prevented.

What is claimed is: 1. A vapor generator which utilizes a primary heated fluid to vaporize a secondary fluid, saidvapor generator comprising a vertical shell portion;.a tubesheet disposed adjacent -one end of. said shell; a tube bundle.

formed from a plurality of tubes through which said primary fluid flows; a wrapper encircling said tube bundle and forming an annular chamber between said wrapper and said shell, .a secondary fluid inlet nozzle; a preheater in fluid communication with said. secondary fluid inlet nozzle and having an end closure plate disposed in a spaced relationship with said tubesheet; a flow distribution plate disposed between said end closure plate and said tubesheet; said flow distribution plate having a plurality of circulation openings disposed therein; said flow distribution plate being cooperatively associated with said end closure plate and said wrapper to form a buffer zone between said end closure plate and said flow distribution plate, said buffer zone preventing influent secondary fluid that is introduced into the preheater from channeling along the tubes and contacting the tubesheet.

2. A vapor generator as set forth in claim 1, wherein the wrapper cooperates with the end closure plate and the flow distribution plate to form a. buffer zone which is enclosed on all sides except one.

3. A vapor generator as set forth in claim 1, wherein"- the end closure plate only has holes which tightly em buffer zone from adjacent tubesheet.

4. A vapor generator 'as set forth in claim 1, wherein tubes are U-shaped.

5. A vapor generator as set forth in claim 1, and further comprising means for separating secondary fluid vapor from the secondary fluid and for directing the separated secondary fluid to the annular chamber, whereby it flows to the space between the tubesheet and the flow distribution plate.

6. A steam generator as set forth in claim 1, wherein the flow distribution plate is generally circular and generally covers the area within the wrapper and the section adjacent the tubes which receive influent primary fluid has a large centrally located opening through which a plurality of tubes pass.

7. A vapor generator as set forth in claim 1, wherein a dividing plate extends upwardly within the wrapper to cooperate with the wrapper to produce a vapor enclosure, which incorporates at least a portion of the tubes which receive influent primary fluid, said vapor enclosure comprising a plurality of baffles which cause the vapor to follow a sinuous path therethrough and become superheated.

8. A vapor generator as set forth in claim 7, wherein the wrapper has openings disposed therein to supply preheated secondary fluid to a portion;o f the annular space between the wrapper and the shell and then to an area between the flow. distribution plate .and the tubesheet. f

9. A vapor generator as set forth in claim 8, and further comprising an outlet nozzle for superheated secondary fluid disposed in the shell and means for sepa-' and the shell. 

1. A vapor generator which utilizes a primary heated fluid to vaporize a secondary fluid, said vapor generator comprising a vertical shell portion; a tubesheet disposed adjacent one end of said shell; a tube bundle formed from a plurality of tubes through which said primary fluid flows; a wrapper encircling said tube bundle and forming an annular chamber between said wrapper and said shell, a secondary fluid inlet nozzle; a preheater in fluid communication with said secondary fluid inlet nozzle and having an end closure plate disposed in a spaced relationship with said tubesheet; a flow distribution plate disposed between said end closure plate and said tubesheet; said flow distribution plate having a plurality of circulation openings disposed therein; said flow distribution plate being cooperatively associated with said end closure plate and said wrapper to form a buffer zone between said end closure plate and said flow distribution plate, said Buffer zone preventing influent secondary fluid that is introduced into the preheater from channeling along the tubes and contacting the tubesheet.
 2. A vapor generator as set forth in claim 1, wherein the wrapper cooperates with the end closure plate and the flow distribution plate to form a buffer zone which is enclosed on all sides except one.
 3. A vapor generator as set forth in claim 1, wherein the end closure plate only has holes which tightly embrace the tubes and the flow distribution plate has holes which encircle the tubes and provide open area adjacent the tubes for secondary fluid to flow into the buffer zone from adjacent tubesheet.
 4. A vapor generator as set forth in claim 1, wherein tubes are U-shaped.
 5. A vapor generator as set forth in claim 1, and further comprising means for separating secondary fluid vapor from the secondary fluid and for directing the separated secondary fluid to the annular chamber, whereby it flows to the space between the tubesheet and the flow distribution plate.
 6. A steam generator as set forth in claim 1, wherein the flow distribution plate is generally circular and generally covers the area within the wrapper and the section adjacent the tubes which receive influent primary fluid has a large centrally located opening through which a plurality of tubes pass.
 7. A vapor generator as set forth in claim 1, wherein a dividing plate extends upwardly within the wrapper to cooperate with the wrapper to produce a vapor enclosure, which incorporates at least a portion of the tubes which receive influent primary fluid, said vapor enclosure comprising a plurality of baffles which cause the vapor to follow a sinuous path therethrough and become superheated.
 8. A vapor generator as set forth in claim 7, wherein the wrapper has openings disposed therein to supply preheated secondary fluid to a portion of the annular space between the wrapper and the shell and then to an area between the flow distribution plate and the tubesheet.
 9. A vapor generator as set forth in claim 8, and further comprising an outlet nozzle for superheated secondary fluid disposed in the shell and means for separating the superheated vapor from the preheated secondary fluid in the annular space between the wrapper and the shell. 